1. Field of the Invention
The present invention generally relates to the filed of an optical disc apparatus and more particularly is directed to an optical disc apparatus for recording data on or reproducing data from an optical disc.
2. Description of the Related Art
Optical disc apparatus require several control circuits in order to precisely record or reproduce data. One of these circuits is a focusing control circuit or a tracking control circuit.
The focusing and tracking control is to direct a focusing error and a tracking error caused by the surface deflection or up-down movement of the disc surface while the optical disc is rotating, and move an optical system such as an objective lens in parallel with and in perpendicular to the optical axis of the laser beam according to the focusing error signal corresponding to the amount of detected focusing error and the tracking error signal corresponding to detected tracking error so as to keep the distance between an optical disc and an objective lens constant and to track a guide groove formed on the optical disc.
In general, the focusing control circuit and the tracking control circuit have an automatic going control circuit.
The automatic control circuit comprises a divider, having an arithmetic amplifier and a multiplier such as a transconductance multiplier connected to a feed back loop portion of the arithmetic amplifier, for keeping an amplitude of the focusing error signal or tracking error signal within a desired range in accordance with a loop gain of the divider. In the case, the loop gain of the divider is changed by controlling the conductance of the multiplier according to the focusing error signal or tracking error signal so as to precisely keeping the amplitude of the focusing error signal or tracking error signal within a desired range.
However, an offset of the automatic control circuit becomes large level in proportional to the loop gain change.
In addition, the amplitude of the focusing error signal obtained by the focusing control circuit and the amplitude of the tracking error signal obtained by the tracking control circuit should be suppressed to predetermined levels so as to remove such control errors.
However, generally, in optical disc apparatus, various types of optical discs produced by different manufacturers are used. The intensity of reflected light of optical discs may differ in the types thereof. The amplitude of the focusing error signal obtained by the focusing control signal and the amplitude of the tracking error signal obtained by the focusing control circuit depend on the intensity of the reflected light.
Thus, in the focusing control circuit and the tracking control circuit, the amplitude factors of the focusing error signal and the tracking error signal should be changed corresponding to the type of an optical disc for use.
A conventional automatic gain control circuit for changing the amplification factors of the focusing error signal and the tracking error signal is as follows;
An input terminal and one input terminal (first input terminal) of an operational amplifier are connected through a first register R. The other input terminal (second input terminal) of the operational amplifier is grounded. The output terminal of the operational amplifier is connected to an output terminal. The output terminal of the operational amplifier and the first input terminal of the operational amplifier are connected through a multiplier and a second resistor Rs. The coefficient of the multiplier is set corresponding to a control signal which is outputted from a control signal input terminal. By changing the coefficient, gain control is performed. In other words, the amplification factors of the focusing error signal and the tracking error signal are changed.
However, the amplification factor of the automatic gain control circuit is roughly given by the following formula. EQU AG=G(Rs+1/gm)/R
where AG is the amplification factor of the automatic gain control circuit; G is a coefficient determined by both the amplification factor of the coefficient of the multiplier; and gm is a conductance of a multiplier. The amplification factor AG is proportional to the offset value of this circuit.
However, the value of 1 /gm varies with a change of the temperature. Thus, the offset value of this circuit drifts in proportional to a change of the temperature. Thus, circuit offset of the focusing error signal and the tracking error signal increases. Thus, control errors of the focusing control and the tracking control result in.